The present invention relates to wireless communication systems, and more particularly, to a universal mobile telecommunication system (UMTS) in which several transmission standards are taken into account by a terminal. These standards include GSM, GPRS and WCDMA.
In a wireless communication system, a base station communicates with a plurality of remote terminals, such as cellular mobile telephones. Frequency division multiple access (FDMA) and time division multiple access (TDMA) are the traditional multiple access schemes for delivering simultaneous services to a number of terminals. The basic idea underlying FDMA and TDMA systems is sharing the available resource (sharing several frequencies for FDMA systems or sharing several time intervals for TDMA systems) in such a way that several terminals can operate simultaneously without causing interference.
In contrast to these schemes using frequency division or time division, the CDMA schemes allow multiple users to share a common frequency and a common time channel by using coded modulation. More precisely, as is well known to the person skilled in the art, a scrambling code is associated with each base station. This makes it possible to distinguish one base station from another. Furthermore, an orthogonal code, known by the person skilled in the art as an OVSF code, is allotted to each remote terminal, such as for a cellular mobile telephone. All the OVSF codes are mutually orthogonal, thus making it possible to distinguish one remote terminal from another.
Before sending a signal over the transmission channel to a remote terminal, the signal has been scrambled and spread by the base section using the scrambling code of the base station and the OVSF code of the remote terminal. In CDMA systems, it is again possible to distinguish between those which use a distinct frequency for transmission and reception (CDMA-FDD system), and those which use a common frequency for transmission and reception but distinct time domains for transmission and reception (CDMA-TDD system).
Third-generation terminals, such as cellular mobile telephones, must be compatible with the UMTS standard, that is, they must be capable of operating under various wireless transmission standards. Thus, they will have to be capable of operating in a system of the FDMA/TDMA type, according, for example, to the GSM or GPRS transmission standard, or else in communication systems of the CSMA-FDD, CSMA-TDD type by using, for example, the UTRA-FDD or UTRA-TDD or IS-95 transmission standards.
It is therefore important to reduce or minimize the number of electronic components in these terminals, so as to reduce their complexity of integration and power consumption to obtain greater autonomy.
In view of the foregoing background, the invention therefore proposes a process for the analog/digital conversion of an analog signal delivered by a reception stage of a terminal of a wireless communication system, for example, a cellular telephone. According to a general characteristic of the invention, the conversion is performed in a delta-sigma type converter, and the parameters of the converter are adjusted on the fly as a function of the transmission standard, the actual rate of transmission of the useful data and the actual conditions of reception.
The invention therefore proposes to use just one converter which is adaptable and flexible. This single converter is a delta-sigma converter having adjustable parameters. Also, since just one analog/digital converter is required to support a number of wireless communication systems, such as CDMA, TDMA, FDMA systems, for example, the complexity of implementation of the terminal is substantially reduced.
Furthermore, the use of a delta-sigma converter also makes it possible to modify the resolution of the converter as a function of the actual rate of transmission of the useful data, that is, the services transmitted. For example, the services transmitted include transmission of data or speech or cell searching in an UTRA transmission standard. The modification of the resolution of the converter can also be adapted as a function of the actual conditions of reception, that is, for example, of the environmental conditions (reception level, type of environment, etc.).
In certain cases it is possible to reduce the resolution of the converter by one or two bits, while satisfying acceptable conditions of reception with regards to the service requested. However, it has been observed that a decrease in resolution by 1 bit could lead to a reduction on the order of 30 to 40% in the consumption of current.
A delta-sigma converter comprises a delta-sigma modulator having adjustable parameters. This modulator includes a sampler, as well as a parametrizable digital output filter connected at the output of the modulator. According to one mode of implementation of the invention, the parameters of the modulator are adjusted as a function of the desired transmission standard. The coefficients of the output filter and the value of the oversampling frequency of the sampler are adjusted as a function of the desired resolution with regards to the actual rate of transmission of the useful data and the actual conditions of reception.
The delta-sigma modulator also generally includes a loop having loop coefficients. This loop includes the sampler and at least one integrator. The adjustment of the parameters of the modulator can then include an adjustment of the loop coefficients and of the value of the oversampling frequency.
Selection between the various resolutions and the various passbands of the signal (corresponding to the various transmission standards), may be controlled by the software used in the digital processing stage which follows the analog/digital converter. By measuring the block error rate, which is a measurement representative of the binary error rate of the signal, it is possible to obtain a good estimate of the environmental conditions. Also, the modification of the resolution and the checking of the new values of resolution can be based on these measurements.
More precisely, according to one mode of implementation of the invention, a measurement representative of the binary error rate of the signal is performed. This measured value is compared with a reference value, which depends for example on the service transmitted, such as speech, low throughput data transmission, etc. Also, if the measured value is less than the reference value, this signifies that there may be a certain margin for decreasing the resolution of the converter.
The parameters of the converter are then adjusted in such a way as to decrease the resolution of the converter in successive notches. Also, at each notch the resolution is checked via a new measurement representative of the binary error rate. Thus, by way of example, if there is an actual rate of transmission of useful data corresponding to a high-quality data transmission service, the reference value of the binary error rate is then specified at 10xe2x88x926. Also, if the converter is tuned for a resolution of 8 bits and should a binary error rate equal to 10xe2x88x928 be measured for example, the resolution can first be reduced to 7 bits. If the new binary error rate is still less than 10xe2x88x926, it is then possible to attempt to reduce the resolution to 6 bits. One generally does not proceed below 4 bits for such a service, until a measured binary error rate, which is compatible with the reference error rate required for this service, is obtained.
The subject of the invention is also directed to a terminal of a wireless communication system, for example, a cellular mobile telephone, comprising a reception stage able to receive a transmitted signal and to deliver an analog signal. An analog/digital converter converts the analog signal to a digital signal, and a processing stage processes the digital signal delivered by the converter.
According to a general characteristic of the invention, the converter is a parametrizable delta-sigma converter, and the processing stage comprises tuning means able to adjust the parameters of the converter on the fly as a function of the transmission standard, the actual rate of transmission of the useful data and the actual conditions of reception.
According to one embodiment of the invention, the delta-sigma converter comprises a parametrizable delta-sigma modulator which includes a sampler, as well as an output filter. The tuning means include first means of adjustment for adjusting the parameters of the modulator as a function of the transmission standard, and second means of adjustment for adjusting the coefficients of the output filter and the value of the oversampling frequency of the sampler as a function of the desired resolution with regards to the actual rate of transmission of the useful data and the actual conditions of reception.
According to one embodiment of the invention, the delta-sigma modulator includes a loop having loop coefficients. The modulator also includes the sampler and at least one integrator. The first means of adjustment are then able to adjust the loop coefficients and the value of the oversampling frequency. The first means of adjustment may also adjust the value of the capacities of the integrators.
According to one embodiment of the invention, the second means of adjustment include a memory storing reference values representative of various reference binary error rates associated with various transmission characteristics, and measurement means able to perform a measurement representative of the binary error rate of the signal.
The second means of adjustment further includes comparison means able to compare this measured value with a reference value, and checking means. If the measured value is less than the reference value, the checking means adjust the parameters of the converter in such a way as to decrease the resolution of the converter in successive notches, and to check at each notch the new resolution on the basis of a new measurement value delivered by the measurement means.